But, there is something we need to remember. Fish are not the
only living beings inhabiting the underwater world. The
underwater world harbours reptiles, insects and plants and
shelters millions of kinds of creatures. The creatures living in
this world have particular methods of eating, breathing and
sleeping peculiar to them.
The respiratory system of fish is different from that of all
other creatures. Instead of noses fish have gills. By means of
gills, they can use the oxygen in water. The water that is
continuously taken into the mouth passes through the gill
arches. Meanwhile, capillaries in the gills take in the oxygen
dissolved in the water and release the carbon dioxide of the
body to the water. Most fish have nostrils but these are not
used for respiration. The nostrils open to tiny sacs by which
fish detect the odour of the water that fills these sacs. Sharks,
for example, find their prey by means of their odours.
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